Logs and lumber are subject to degradation from molds and sapstaining fungi during storage and shipment. Molds grow on the surface of wood and cause discoloration through the mass production of spores. Sapstaining fungi penetrate deeply into sapwood through their dark pigmented hyphae and stain wood to black, blueish or grey appearance. Customers for lumber place a high value on the appearance and quality of the product they receive. Wood stain can significantly reduce the value of the lumber by lowering its grade, resulting in monetary and market losses. To offset fungal discoloration, lumber is kiln dried or chemically treated after processing in sawmills. Although a growing amount of wood is kiln-dried, chemical treatment is still the main process for wood stain control for export lumber (Gilbert, 1988). Most of the chemicals in use are synthetic compounds and have a broad spectrum of activity. Public and government concern for the environment has put use of chemicals in sawmills under increasing scrutiny, and there remains an interest in exploring new products and technology for wood protection.
A better understanding of factors affecting growth of molds and staining fungi helps to develop rational chemical or biocontrol agents or develop physical protection methods against these fungi. In a practical sense, when the critical growth requisites of a wood-degrading fungus are known, it is sometimes feasible to modify wood handling or use practices which adversely affect fungus growth, thereby achieving prevention or economic control of the problem.
Like all living organisms, fungi need certain requirements for growth and survival. The major factors that affect the growth of wood-inhabiting fungi are substrate (food), water (above the fiber saturate point, greater than 20% water in wood), oxygen (more than 20% of the wood-void volume), a favorable temperature range, and a suitable pH value (Brock et al., 1988; Zabel & Morrell, 1992). Although many of these factors have been well studied, the effect of pH on the growth of wood staining fungi is not well documented.
Hydrogen ion concentration (pH) of the substrate is a main factor affecting substrate availability, exoenzyme stability, cell permeability and solubility of minerals and vitamins to microorganisms. It is widely acknowledged that fungi generally dominate in acidic environments (pH 3-6) although a wide range of pH (1.5-9.0) is often tolerated by wood inhabiting fungi (Kaarik, 1974; Alexander, 1977). Most wood decay basidiomycetes grow only at low pH values, and the brown rots (basidiomycetes) are particularly sensitive to high pH and often do not grow at pH above 6.5 (Kaarik, 1974). Molds and sapstaining fungi are more pH tolerant and may occur between pH 2 to 10 (Panasenko, 1967; Land et al., 1987). The growth of these wood-inhabiting fungi in wood can be reduced by alkali treatment (Bergman et al., 1970).
The use of biocontrol agents to protect wood from fungal degradation has been recognized as a possible alternative to chemical treatment and has received intensive investigation worldwide for many years (Freitag et al., 1991; Bruce & Highley, 1991; Stranks, 1976). In this strategy, fungi or bacteria that do not discolor wood are introduced onto lumber after cutting. These organisms grow on the wood surface and protect wood against attack by stain or decay fungi. This approach has worked well in laboratory testing for many microorganisms, but there has been limited field success (Morris et al., 1984; Morrell & Sexton, 1990).
Gliocladium roseum Bainier, also known as Gliocladium aureum Rader, has been identified as a potential biocontrol agent against sapstain on small blocks of softwood in laboratory tests (Seifert et al., 1988). This antagonist protected pasteurized lumber from stain (McAfee & Gignac, 1996), but showed inconsistent performance on green lumber in field testing (McAfee & Gignac, 1997). The poor performance of this fungus in lumber under natural conditions is due to the lack of a comprehensive competitive ability against other wood-inhabiting fungi. To improve the effectiveness of a biocontrol agent, one approach could be to selectively alter conditions of wood with a view to giving the promoting fungus a competitive advantage (Morrell & Sexton, 1992; Dawson-Andoh & Morrell, 1992).